Narcolepsy is a frequent disorder: it is the second leading cause of excessive daytime sleepiness diagnosed by sleep centers after obstructive sleep apnea. Studies on the epidemiology of narcolepsy show an incidence of 0.2 to 1.6 per thousand in European countries, Japan and the United States, a frequency at least as large as that of Multiple Sclerosis. In many cases, however, diagnosis is not made until many years after the onset of symptoms. This is often due to the fact that patients consult a physician after many years of excessive sleepiness, assuming that sleepiness is not indicative of a disease.
Narcolepsy is a very disabling and underdiagnosed illness: the effect of narcolepsy on its victims is devastating. Studies have shown that even treated narcoleptic patients are often markedly psychosocially impaired in the area of work, leisure, interpersonal relations, and are more prone to accidents. These effects are even more severe than the well-documented deleterious effects of epilepsy when similar criteria are used for comparison.
The large majority of narcoleptic patients in this country are still undiagnosed, and narcoleptic subjects are most often diagnosed only after many years of struggle. In one recent study, the mean number of years between the onset of symptoms and correct diagnosis was 14 years. Since the symptoms of narcolepsy usually appear during adolescence, this means that most narcoleptic patients are diagnosed too late to prevent the dramatic impact of the disease on their personal and professional development.
The main symptoms of narcolepsy are excessive daytime sleepiness and abnormal REM sleep: Narcolepsy is not only a serious and common medical problem, it also offers basic sleep researchers a unique opportunity to gather new information on the central mechanisms regulating REM sleep and alertness. Since the 1960s it has been known that several of the disabling symptoms of narcolepsy, such as sleep paralysis, cataplexy and hypnagogic hallucinations, are pathological equivalents of REM sleep. In sleep paralysis, a frightening symptom considered to be an abnormal episode of REM sleep atonia, the patient suddenly finds himself unable to move for a few minutes, most often upon falling asleep or waking up. During hypnagogic hallucinations, patients experience dream-like auditory or visual hallucinations, while dozing or falling asleep. Cataplexy, a pathological equivalent of REM sleep atonia unique to narcolepsy, is a striking, sudden episode of muscle weakness triggered by emotions. Typically, the patient's knees buckle and may give way upon laughing, elation, surprise or anger. In other typical cataplectic attacks the head may drop or the jaw may become slack. In severe cases, the patient might fall down and become completely paralyzed for a few seconds to several minutes. Reflexes are abolished during the attack.
Narcolepsy can be diagnosed using specific medical procedures: the diagnosis of narcolepsy is usually easy if all the symptoms of the illness are present. More often, however, the symptoms of dissociated REM sleep such as cataplexy are mild, and a nocturnal polysomnogram, followed by the multiple sleep latency test (MSLT) is suggested. This test, performed at a sleep disorders clinic, will confirm the daytime sleepiness by showing a short sleep latency of usually less than 5 minutes, as well as an abnormally short latency prior to the first REM period (SOREMPs). Other causes of daytime sleepiness, such as sleep apnea or periodic leg movements, are also excluded by the nocturnal recordings.